Inorganic glass coating and method for making



LOU U 3,460,955 INORGANIC GLASS COATING AND METHOD FOR MAKING Ellis JohnAirola, Lynn, Mass., assignor to General Electric Company, a corporationof New York No Drawing. Filed Nov. 22, 1965, Ser. No. 509,158 Int. Cl.C23f 11/00; C09d 1/02 U.S. Cl. 106-74 8 Claims ABSTRACT OF THEDISCLOSURE An inorganic phosphate material of improved corrosionresistance and stability and useful as pigmented or unpigmented coatingmaterial is the reaction product of an aqueous mixture of phosphoricacid, chrowacid, acidic W01, magnesium and magnesium phosphate dibasicwhen the ratio of the phosphoric acid to the magnesium phosphate dibasicis at least 1. One pigmented form includes finelygiyidegwinum.

This invention relates to inorganic coatings and, more particularly, toan inorganic phosphate-silicate glass coating and method for making.

Metallic pigmented coatings including binders of organic resins orsilicones are limited in their use by the decomposition temperature ofsuch binders. Generally above such temperatures as about 600 F., thebinders decompose leaving the pigment, silica if a silicone is includedand perhaps varying amounts of carbon. If any kind of bond remainsbetween the coating and the coated article, it is based primarily uponattraction between the pigment and the base article. It is necessary toraise the temperature considerably above 600 F. to achieve anysuccessful degree of diffusion bonding between the metallic pigment,frequently aluminum,-and the base metal. However, processing of articlessuch as blades for axial flow compressors at certain elevatedtemperatures can reduce mechanical properties and can change accuratedimensions because the articles may warp or bend.

Because the inorganic phosphate glasses have somewhat higher temperatureresistance than organic resins or silicones, coatings using phosphateglasses as a principal ingredient-have been selected for use such asblade coatings because they can be cured at temperatures in the range ofabout 600-800". Whereas these coatings provide a base article with somecorrosion resistance, much greater resistance to corrosion is requiredin many applications' In addition, these kinds of coatings are diflicultto apply smoothly in very thin coatings and when applied in a thin layerhave relatively poor corrosion resistance. Furthermore, it has beenfound that the corrosion resistance of available coating materials ofthis type will be afiected detrimentally by long shelf life.

It is a principal object of this invention to provide an improvedinorganic phosphate-silicate glass material which is easily applied as athin smooth, adherent coating in the pigmented or unpigmented conditionand which has improved corrosion resistance.

Another object is to provide a method for making such an improvedmaterial.

These and other objects and advantages will be more readily understoodfrom the following detailed description and examples which are meant tobe typical of rather than any limitation on the scope of the presentinvention.

It has been found that the objects of this invention can be achievedwith a pigmented material which is the reaction product of a mixture of,by weight, about 8% to less than 12% phosphoric acid (calculated as 85%United States Patent Patented Aug. 12, 1969 H PO more than 1.3 up toabout 5% chromic acid; about 0.4% to less than 0.9% magnesium; about 8%to less than 12% magnesium phosphate dibasic about 15-30% acidic sodiumsilicate sol (calculated as 34% Si0z); 13-30% water with the balancefinely divided aluminum, provided the ratio of the phosphoric acid tothe magnesium phosphate dibasic is 'at least 1. In its unpigmented form,the material is the reaction product of a mixture of, by weight, about13% to less than 16.7% phosphoric acid more than 2.1 up to about 8.5%chromic acid; about 0.6% to less than 1.5% magnesium; about 12% to lessthan 19.5% magnesium phosphate dibasic; about 25-42% acidic sodiumsilicate sol (34% SiO and about 22-45% water.

One method by which the material of the present invention can be madecomprises the steps of first reacting together the chromic acid, water,phosphoric acid and magnesium. Then the magnesium phosphate dibasic isdissolved in the solution. The solution is cooled and the acidic sodiumsilicate sol is added followed by the pigment. In a specificallypreferred form, all of the magnesium is reacted with suflicientphosphoric acid, chromic acid, and water in one container. All of theadded magnesium phosphate dibasic is dissolved in another container withthe balance of the chromic acid, phosphoric acid and water. Then theproducts of reactions from the two containers are mixed together afterwhich the acidic sodium silicate sol is added followed by the pigment.

The material of the present invention combines the characteristics ofimproved corrosion resistance even when applied as a thin coating, alongwith ease of application as a smooth, adherent heavy or thin coating.Normally sodium silicate will tend to be unstable in an acid solutionsuch as exists with the presence of phosphoric acid. However, it hasbeen found that if the sodium silicate is added as an acidic sol, astable solution results. This solution changes to a thixotropic stateafter about 8-10 weeks of standing. With aluminum as a pigment, thismaterial has unusually good corrosion resistance and results in a smoothsurface when applied as a coating.

A review of some of the typical exa-inples of materials and methods formaking materials suitable for coatings which were evaluated in thedevelopment of the present invention will provide a clearerunderstanding of the various aspects of the present invention. Theexamples given are typical of but are not meant to be any limitation onthe scope of the present invenion.

As a basis for comparison with regard to corrosion resistance, acommercially available aluminum pigmented coating material includingphosphoric acid and phosphate compounds along with about 40 weightpercent aluminum and 40 weight percent water was obtained. An analysishas shown that about 19 weight percent of this material is some type ofphosphate, with no silicates present. This material has been used as acoating in thicknesses of between 2 and 3 mils to provide corrosionresistance for certain larger components of gas turbine engines whichcan tolerate heavier coatings without disturbing aerodynamiccharacteristics. However, corrosion resistant coatings are needed forsmall components such as the blading members of small axial flowcompressors in which a coating thickness of 2-3 mils couldseriouslyatiect air flow and eiriciency. Consequently, the presentinvention was evaluated, as far as corrosion resistance is concerned, asa coating of about 1.5 mils or less. For purposes of this description,the standard commercial material will hereinafter be referred to asExample SC.

The following Table I summarizes one series of materials formulated inthe evaluation of the present invention.

The .ingredients of Example 1 had a relatively high phosphoric acid andmagnesium content to promote the formation of magnesium phosphate in theabsence of any added magnesium phosphate dibasic as a separate chemicaland to produce chromium phosphate. However, a precipitation of silicicacid occurred upon the addition of the acidic sodium silicate sol.Furthermore, the binder portion of the mixture of Example 1, as didExample 2, gelled after a shelf life of about 3-7 days. The mixture ofExample 2 blistered upon curing at 200 F. and was found to require toomuch acid to produce the magnesium and chromium phosphates. Therefore,magnesium phosphate dibasic was added in Example 3. However, the ratioof phosphoric acid to magnesium, phosphate dibasic in Example 3 was toolow (less than 1) to allow all of the magnesium phosphate dibasic todissolve. As shown by Example 4, the larger amount of phosphoric acidrequired to bring such ratio to 1 in order to dissolve all of themagnesium phosphate dibasic resulted in an unstable binder which gelledafter 6 days because of the high phosphoric acid content. Thus it wasfound that within the range of this invention 12 weight percentmagnesium phosphate dibasic is excessive, and that amounts of phosphoricacid at levels of 12 weight percent or more in the pigmented materialwill cause early binder gelling of the non-thixotropic type.

It was recognized that gelling of the binder is a function, at least inpart, of the phosphoric acid content. Therefore, with the exception ofExample 7 shown in subsequent tables, the total phosphoric acid contentwas mamtained at less than 12 weight percent in the pigmented materialof the present invention and less than about 16.7 weight percentcalculated on the basis of unpigmented binder.

The composition of Examples and 6 were made by first mixing together allof the required chromlc acid, water, phosphoric acid and magnesium inorder to produce first a solution of magnesium and chromium phosphatesas the-reaction product of such mixture. The resulting reaction is anexothermic one which raised the temperature of the solution to about 180F. and dissolved all of the ingredients mixed. After these firstingredients were all in solution, the magnesium phosphate dibasic wasadded and dissolved. Then the solution was cooled to room temperatureafter which the acidic sodium silicate sol was added followed by thealuminum.

The pigmented reaction products which resulted from the mixture ofExamples 5 and of 6 were sprayed as coatings of 0.9-1.2.mils thicknessonto a series of 2" x 5" test panels of A151. type 410 stainless steel.

The coatings were applied by spraying with a standard paint spray 81m toproduce the desired thicknesses. Then the panels were air dried for 15hour, heated at 200 F. for A hour and then heated for 16 hour at 800 F.before cooling to room temperature. All of the heating steps wereconducted in an air atmosphere oven.

These panels were subjected to a standard ASTM salt fog test along withpanels of the standard commercial product (Example SC) coated to thesame thickness and used as control panels. After three hours, the panelscoated with Example SC displayed many corrosive sites. After 168 hoursexposure, the panels coated with the Example 5 reaction product stillhad a smooth, adherent surface and no corrosion sites. When Example 5coated panels were examined after 240 hours exposure only one of thepanels had developed a single corrosion site. Similarly, panels coatedwith the reaction product of the mixture of Example 6 showed nocorrosion sites aftcr 168 hours exposure and the start of some corrosionupon inspection at 240 hours. As was mentioned above, the control panelscoated to the same thickness with Example SC developed many corrosionsites in 3 hours.

By maintaining the proper balance of the ingredients as shown byExamples 5 and 6 and by mixing these ingredients as was described inconnection with those examples, a reaction product material whichprovides a smooth, adherent and unusually corrosion resistant barrier orcoating can be produced. It has been found that the reaction product ofthe present invention can be deposited with an ordinary paint spray gunin smooth coatings as thin as about $6 mil and still provide corrosionresistance many times that of the commercially available materials ofthis type. The present invention provides a stable silicate bearingphosphate glass coating material which maintains its stability evenunder acidic conditions.

In Examples 1 through 6, the chemicals used were chromic acidFischerA-lOO, magnesium tumings- Baker, purified No. 2420, ortho phosphoricacid Fischer Ae-242, acidic sodium silicate sol (34% SiO,) NalcoChemical Co. (Nalcoag 1034A), magnesium phosphate dibasic powder(MgI-IPOy3H O)Mallinckrodt and aluminumReynolds Metal powder, atomized,6 micron diameter and containing no lubricants. However, it was foundthat by reacting the magnesium with part of the chromic acid, phosphoricacid and water in a container separate from one in which the magnesiumphosphate dibasic was being dissolved by the balance of the chromic andphosphoric acids with the water, the respective reactions and solutionscan be more easily obtained. Therefore, it is preferred that themagnesium metal and part of the chromic acid, phosphoric acid and waterbe mixed in one container. The magnesium phosphate dibasic chemical andthe remainder of the phosphoric acid, chromic acid and water are mixedin a second heated container in amounts sufficient to place all of themagnesium phosphate dibasic into solution. According to this method ofmixing and reacting of ingredients separately, all of the magnesiumphosphate and chromium phosphate produced in the first container will bemaintained in solution when the products of the first and secondcontainers were mixed together. Chromic acid in excess of that requiredin the reaction is included in the material of the present invention toprotect the aluminum from being attacked by phosphoric acid. ,After theingredients of the first and second containers are mixed together, theacidic sodium silicate sol was added after which the aluminum wasintroduced. 1

In order to further evaluate this preferred method form of producing thematerial of the present invention, a number of examples were studied.Typical of these examplese were those shown in Table II which isseparated into several groupings. The first group of materials weremixed in a first container and the second group in a second container.The acidic sodium silicate sol was added afterthe reaction products ofthe first and second groups were intermixed. Then the aluminum pigmentwas added.

In order to more easily visualize the total amounts of chemicals mixed,the following Table III summarizes the total ingredients of the examplesof Table II.

[Ingredients (wt. percent)] Examples HaPOr/MgHPOMHzO 1.4 1.2 1.2 1.3 1.01.1 1.0 1.0 1.0 1.0

TABLE 111 [Total ingredients (wt. percsnt)] Examples Crr---... 2.2 2.21.3 2.7 4.5 5 5 5 H2O 25.2 24.2 25.3 25.8 22.5 24.1 23.6 18.6 13.6 HsPO4(85%)..--- 9.2 9.7 8.1 9 8 8 8 8 MgHPOq-BHgO 8 8 8 8 8 8 8 8 8 8 M 0.50.4 0.4 0.9 0.4 0.4 0.5 0.4 0.4 0.4 0.4 15 15 15 15 15 15 15 25 33.4 4040 40 40 40 40 40 40 IhPOi/MgHPO4-3H2O 1.4 1.2 1.2 1.3 1.0 1.1 1.0 1.01.0 1.0

The mixture of Example 7 was made to determine if product of Example 8,with a total phosphoric acid content of 11 weight percent and aphosphoric acid to magnesium phosphate dibasic ratio of 1 or greater,resulted in an excellent product which sprayed well. There was nocracking or blistering of the coating after the 800 F. cure. Similarly,the material of Example 9 sprayed well and had the same good adhesionafter curing.

Exampe 10 represented an attempt to increase the amount of chromiumphosphate formed in the first container through the addition of anincreased amount of magnesium to assist in that reaction. However, thefinal product, after all of the ingredients had been added, hecamegrainy and then grannular within three days, although the binder did notgel. Therefore, the magnesium added in the mixture which results in theproduct of the present invention should be less than about 0.9 weightpercent in order to maintain the necessary balance of reaction products.

As was mentioned before in connection with Examples 1 through 4, theinclusion of 12 weight percent or more total phosphoric acid (85%)within the range of the material of this invention produces a binderwhich forms a non-thixotropic gel within a short period of time.However, a careful balance between phosphoric acid and chromic acid mustbe maintained so that too much of an excess of phosphoric acid does notremain after reaction in the first container. This was substantiated bythe reaction product of Example 11 which gelled within a few days,although the product when sprayed immediately after mixing had a coatinglayer which was smooth and tightly' adherent after curing. Thereforemore than a total of 1.3 weight percent chromic acid should be includedin the material of this invention, to assist the solution of themagnesium phosphate dibasic within the range of the present invention.

The ingredients of Examples 12 through 15 result in satisfactoryreaction products. Corrosion testing in the" standard ASTM salt fogchamber of the reaction product of Example 12 after spraying tothicknesses of between 0.45 and 0.75 mil and curing at 800 F. shown inthe Table IV.

TABLE rv.-oonRos10N DATA Time (in hours) to first 0;]!1'081011 siteCoating and number 0! tes thickness Example (mils) 2 24 40 44 46 48 64Many.

Similarly, the reaction product of Example 13 coated to thicknesses of0.70 and 0.85 mil after being subjected to the standard ASTM salt fogtest showed no corrosionthe panels coated with the reaction product ofExample In order to show more clearly the efiect of the addition ofacidic sodium silicate sol on the reaction product formed by the mixtureof the first and second groups of materials of Table II, along with theinclusion of aluminum, a series of evaluations were made at varyinglevels 8 acidic sodium silicate sol compared with 15% or less inExamples 16-19.

After months of storage, the binder of Example 21, to which the aluminumhad not been added, had formed into a thixotropic state. Upon mildagitation, the gel reverted to its original fluid condition. Thenaluminum pigment was added and test panels were prepared as de- TABLE VWeight Percent Sodium Binder Silicate Na Silicate scribed before. TableVII summarizes corrosion data ob- Al lnblnder Example tained from thisbatch of material having the composition of Example 21 including 25%acidic sodium silicate sol.

TABLE VII.-CORROSION DATA [Example 2125% acidic Na silicate sol] Time inhrs. and accumulative total The following Table VI shows the conditionof Examples 16-20 of panels coated to various thicknesses and a cured asindicated above at 800 F.

sites exclusive of edge effects Panel N o. 466

TABLE VL-CORROSION DATA [020% acidic sodium silicate sol] Coating Timesin hours and number of sites Thickness (mils) 15 Example 012 MJLL 00Observations were made at 8 hours and thereafter daily. Aside from someslight edge effects from the uncoated panel edges, no corrosion siteswere observed before 360 ammmwawmmwwemnaw Q00 0 0 QQ0 0 hours. A singleedge effect was first noted at 264 hours for the 0.60 mil coating. Evenafter 500 hour fects were very slight s, the edge efbeing non-existenton Panels 4, 6

and 8. At 500 hours, only the 0.60 mil coating showed Some evidence ofdecreasing corrosion resistance. It is im- It is noted that at theinclusion of more than 15 weight percent acidic sodium silicate sol, thecorrosion resistance portant to note that the control panels coated to athickness of 0.5-1 mil with Example SC material were marked withnumerous corrosion sites after only about 3 hours under the sameconditions.

1 Many.

In order to more clearly show the relationship between the ingredientsincluded in the binder alone, the percentage by weight of the binderingredients in the examples are shown in Table VIII.

TABLE VIIL-UNPI GMENTED BINDER [Ingredients (wt. percent)] of a curedcoating is significantly improved. Thus the Sodium silicate solutionsnormally are readily decomacidic sodium silicate sol content added tothe reaction posed by acids with the precipitation of silicic acid.Howproduct of the first and second containers or added to the ever, thepresent invention provides a material which, beingredients shown inExamples 5 and 6, when used with cause of its balance of ingredients andreaction products,

a pigment, is at least about 15 weight percent. On the can and doesretain a substantial quantity of sodium basis of the pigment free binderalone, as shown later in silicate in an acid medium. The resultantstable product following Table VIII the acidic sodium silicate sol isincan be used as a thin corrosion inhibiting barrier for such cluded inthe range of at least about 25 weight percent. materials as those basedon iron.

It has been shown by the above data that the inclusion Although thepresent invention has been described in of at least 15 weight percentacidic sodium silicate sol' connection with a number of specificexamples, it will be results in significantly better corrosionresistance. How understood by those skilled in the art the variationsand .ever, variations in this material when obtained com-' modificationsof which the present invention is capable mercially can result invariations in corrosion resistance such as the possible substitution ofthe ingredients at difat about the 15% level. As shown by the data ofTables ferent concentrations which will modify the percentages VI andVII based on the compositions of Table V, it is or of equivaentingredients which perform functions the preferred that the acidic sodiumsilicate sol be included same as those shown in the examples. Forexample, at more than 15% and preferably about 20-25%. Note MgHP0 -3H 0form of magnesium phosphate was used in Table VI that significantlyimproved corrosion resistbecause it is one of the more readily solubletypes of ance results from the inclusion in Example 20 of 20% magnesiumphosphate.

What is claimed is:

1. An improved inorganic phosphate material useful as a binder for acorrosion resistant coating, the material being the reaction product ofa mixture consisting essentially of, by weight:

Percent Phosphoric acid when calculated as 85% HgPO; to Chromic acid 2.1to 8.5 Magnesium -a 0.6 to 1.5 Magnesium phosphate dibasic 12 to (19.5Acidic sodium silicate sol when calculated as 34% SiO 25-50 and Water-45 the ratio of the phosphoric acid to the magnesium phosphatedibasicbeing at least 1. 2. The improved material of claim 1 in whichthe mixture consists essentially of, by weight:

Percent Phosphoric acid when calculated as 85% H PO 13-16 Chromic acid3-8 Magnesium 0.6-0.8 Magnesium phosphate dibasic 13-16 Acidic sodiumsilicate sol when calculated as 34% SiO, 33-42 and Water 22-45 the ratioof the phosphoric acid to the magnesium phosphate dibasic'being at least1.

3. An improved pigmented inorganic phosphate coating material which isthe reaction product of a mixture with the balance finely dividedaluminum;

the ratio of the phosphoric acid to the magnesium phosphate dibasicbeing at least 1.

4. The improved pigmented material of claim 3 in which the mixtureconsists essentially of, by weight:

- 1 Percent Phosphoric acid when calculated as 85 -H,PO 8-10 Chromicacid 2-5 Magnesium 0.4-0;5 Magnesium phosphate dibasic 8-10 Acidicsodium silicate sol when calculated as 34% SiO, 20-25 Water 2040 withthe balance finely divided aluminum;

the ratio of the phosphoric acid to the magnesium phosphate dibasicbeing at least 1.

5. A method for making an inorganic phosphate material useful as abinder for a corrosion resistant coating, comprising the steps of:

first mixing together, by weight, more than 2.17 to 8.57 chromic acid,20-457 water, 13% to less than 16.7% phosphoric acid calculated as 85% HPO and 0.6% to less than 1.5% magnesium until the reaction producing areaction product is completed;

dissolving in the reaction product 12 to less than 19.5

7 weight percent magnesium phosphate dibasic;

' cooling the resulting solution to about room temperature; and thenadding 25-50 weight percent acidic sodium silicate sol calculated as 34%SiO,.

6. A method for making a pigmented inorganic phosphate material in whichaluminum pigment is included, comprising the steps of:

first making a mixture by (a) making a reaction product by mixingtogether, by weight, more than 2.1% to 8.5% chromic acid, 20-45% water,11% to less than 16.7% phosphoric acid calculated as H PQ and 0.6% toless than 1.5% magnesium until (the reaction producing the reactionproduct is completed;

. (b) dissolving in the reaction product 12 to less than 19.5 weightpercent magnesium phosphate dibasi c;

(c) cooling the resulting solution to about room temperature; and

(d) adding 25-50 weight percent acidic sodium silicate sol calculated as34% Slo to complete the first mixture; and then adding linelydividedaluminum to the mixture to make the pigme'hted inorganic phosphatematerial consisting essentially of, by weight:

. Percent Phosphoric acid when calculated as HPO 8 t0 Chromic acid 1.3to 5 Magnesium 0.4 to 0.9 Magnesium phosphate dibasic 8 to 12 Acidicsodium silicate sol when calculated as 34% SiO, 15-30 13-30 Water withthe balance finely divided aluminum; the ratio of the phosphoric acid tothe magnesium phosphate dibasic being at least 1. 7. A method for makingfrom ingredients an inorganic phosphate mate'rial useful as a binder fora corrosion resistant coating, comprising the steps of:

making a reaction product from a first portion of the ingredients, thefirst portion consistingessentially of, by weight; based on totalingredients, 1.1-1.8% chromic acid, 10-20% water, about 6.5% phosphoricacid calculated as 85% H PO and 0.6% to less than 1.5% magnesium to formthe reaction product;

making a solution from a second portion of the ingredients, the secondportion consisting essentially of, by weight based on total ingredients,more than 1% up to 6.7% chromic acid, 16-25 water, 6.5 10% phosphoricacid calculated as 85% HgPO and 12% to l s than 19.5% magnesiumphosphate dibasic to 5211 the solution;

mixing together the reaction product and the solution;

cooling to about room temperature; and then adding 25-50 weight percentbased on total ingredignts, of acidic sodium silicate sol calculated as34% 8. A method for making from ingredients a pigmented inorganicphosphate material in which aluminum pigment is included, comprising thesteps of:

making a reaction product from a first portion of the ingredients, thefirst portion consisting essentially of, by weight based on totalingredients 0.7-1% chromic .acid, 6-11% "water, about 4% phosphoric acidand 0.4% to less than 0.9% magnesium; making a solution from a secondportion of the ingredients, the second portion consisting essentiallyof, by weight based on total ingredients, more than 0.6% up to 4%chromic acid, 10-20% water, 4-7% phosphoric acid, and 8% to less than12% magnes ium phosphate dibasic; mixing together the reaction productand the solution; cooling to about room temperature;

adding 15-30 weight percent, based on total ingredients acidic sodiumsilicate sol calculated as 34% 2,998,328 8/1961 Munger et al "106-84810,; and then 3,130,061 4/1964 McMahon et a1.

adding 33-40 weight percent based on total ingredi- 3,214,302 10/1965Brodt et al 10674 ents finely divided aluminum.

References Cited 5 JAMES E. POER, Primary Examiner UNITED STATES PATENTSs, 1, 2,529,206 11/1950 Winslow et a1 106-74 106-84 2,807,552 9/1957Robinson et a1 106-74

